Polyatomic ions are defined as a group of atoms that contain a charge. 

They are typically clusters of atoms that act as an ion, which carry a fixed charge on them.

Some examples of polyatomic ions are ammonium ${\mathrm{NH}}_4^{+}$, Hydroxide- ${\mathrm{OH}}^{-}$, and Nitrate- ${\mathrm{NO}}_3^{-}$_.